EP0005046B1 - Electrical alarm circuits and systems - Google Patents

Electrical alarm circuits and systems Download PDF

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Publication number
EP0005046B1
EP0005046B1 EP19790300631 EP79300631A EP0005046B1 EP 0005046 B1 EP0005046 B1 EP 0005046B1 EP 19790300631 EP19790300631 EP 19790300631 EP 79300631 A EP79300631 A EP 79300631A EP 0005046 B1 EP0005046 B1 EP 0005046B1
Authority
EP
European Patent Office
Prior art keywords
alarm
signalling
line
electrical
circuit
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
EP19790300631
Other languages
German (de)
English (en)
French (fr)
Other versions
EP0005046A1 (en
Inventor
Trevor John Woolvin
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Chubb Electronics Ltd
Original Assignee
Chubb Electronics Ltd
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Filing date
Publication date
Application filed by Chubb Electronics Ltd filed Critical Chubb Electronics Ltd
Publication of EP0005046A1 publication Critical patent/EP0005046A1/en
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Publication of EP0005046B1 publication Critical patent/EP0005046B1/en
Expired legal-status Critical Current

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    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B25/00—Alarm systems in which the location of the alarm condition is signalled to a central station, e.g. fire or police telegraphic systems
    • G08B25/01—Alarm systems in which the location of the alarm condition is signalled to a central station, e.g. fire or police telegraphic systems characterised by the transmission medium
    • G08B25/018—Sensor coding by detecting magnitude of an electrical parameter, e.g. resistance

Definitions

  • This invention relates to electrical alarm circuits and electrical alarm systems including such circuits, being concerned especially with electrical alarm circuits in which there is electrical-signal change in a first or second of two signalling lines in dependence respectively upon whether a first or second alarm condition exists.
  • Electrical alarm systems for use in providing alarm signals in response to intrusion into an area to be protected also conventionally include provision for responding to attempts to interfere with or otherwise tamper with, the intrusion-detector units of the system.
  • the cabling that is provided to connect the detector units to the relevant central station or alarm-control unit incorporates three pairs of lines, a first pair of lines being used for signalling the intrusion-alarm condition, a second pair for signalling the tamper-alarm condition, and the third pair for supplying electrical power to the detector units.
  • These lines which may extend over considerable distance, are vulnerable to attack and unless expensive precautions are taken for their protection can represent a weakness in the security of the alarm system as a whole. It is an object of the present invention to provide an electrical alarm circuit which may be used to improve security in this respect.
  • Such system which, is for use in indicating a fault condition of an anti-skid braking control system, includes a fault-indication unit that is connected remotely to the control system via the single signalling line and a power-supply line with ground return.
  • the signalling line is connected in the anti-skid control system to the collector-emitter current path of a transistor so as to be clamped at a substantially constant potential while the transistor remains in its normally non-conductive condition. This transistor is caused to conduct, thereby changing the potential of the signalling line from its normal, clamped value, whenever a fault condition occurs in the control unit.
  • a transistor or SCR in the fault-indication unit is held non-conductive by the normal, clamped potential of the signalling line, and responds to the change in this potential brought about by the fault condition, to conduct and thereby activate the fault-indication unit to break power supply to the anti-skid braking system and to indicate the alarm.
  • the potential of the signalling line also departs from the normal, clamped value if that line is broken, so that the fault-indication unit is similarly activated and gives the same alarm indication, in these circumstances also.
  • an electrical alarm circuit of the kind in which there is electrical-signal change in a first or second of two signalling lines in dependence respectively upon whether a first or second alarm condition exists, characterised in that the first signalling line is connected to a device which is operative in one or the other of two current-conducting modes in series with the second signalling line in dependence upon whether the first alarm condition exists, that substantially the same magnitude of current flow is established in the second signalling line via said device in the two current-conducting modes, that the magnitude of current flow in the first signalling line via said device is dependent upon which of the two current-conducting modes is applicable, and that occurrence of the second alarm condition is effective to change the overall magnitude of current flow in the second signalling line whereby the magnitudes of current flow in the first and second signalling lines are indicative of the existence or otherwise of the first and second alarm conditions respectively.
  • the circuit may simply include a transistor having base and emitter electrodes connected to the first and second signalling lines respectively, and arranged such that the collector circuit of the transistor is interrupted-for example by the opening of a set of switch contacts-in response to the occurrence of the first alarm condition so as to switch main current flow within the transistor from the collector-emitter path to the base-emitter path and thereby increase current flow in the first signalling line.
  • the emitter current of the transistor may be utilized in these circumstances to establish current flow in the second signalling line that remains substantially constant irrespective of the occurrence of the first alarm condition, but which is interrupted or otherwise changed if one or both of the signalling lines is broken or they are shorted together. Accordingly it is possible with the electrical alarm circuit of the present invention to provide distinctive indications of intrusion-alarm and tamper-alarm conditions in a simple and effective manner, and to do this with just the two signalling lines in addition to any power-supply lines required.
  • the drawing shows an electrical intrusion-alarm system with the electrical alarm circuits incorporated into intrusion-detector units of the system.
  • the alarm system is shown as comprising three detector units 1 that are linked via a multi-core cable 2 with a central, control unit 3.
  • the detector units 1 incorporate conventional detectors 4 that are located and arranged to respond to intrusion into the area to be protected; the detectors 4, which may be active or passive devices, respond not only to intrusion but also to attempts to tamper with their operation.
  • each detector 4 includes one set of normally-closed contacts 5 that are opened whenever an intrusion alarm is to be given, and a set of normally-closed contacts 6 that are opened whenever a tamper alarm is appropriate.
  • the opening of either set of contacts 5 and 6 is signalled via the cable 2 to control equipment 7 of the unit 3.
  • the equipment 7 processes such signals in accordance with normal practice, to provide the required alarm procedures and responses.
  • the cable 2 linking the units 1 with the unit 3 involves just two pairs of leads.
  • the leads of one pair provide direct- current supply-lines 10 and 11 to the units 1 from the positive and negative terminals 12 and 13 respectively of the 12-volt power supply for the unit 3.
  • the other pair of leads provide lines 14 and 15 interconnecting the circuits 8 and 9 for signalling intrusion-alarm and tamper-alarm conditions to the unit 3.
  • Each circuit 8 includes an N-P-N transistor 16 having its base electrode connected directly to the line 14 and its collector electrode connected to the line 10 via a diode 17 (protecting the transistor 16 against supply-polarity reversal) and the contacts 5 of the respectively- associated detector 4.
  • the emitter electrode of the transistor 16 is connected to the line 15 via a resistor 18. Connection in parallel with the collector-emitter path of the transistor 16 is also made to the line 15 from the line 10 via a resistor 19 that is connected in series with the contacts 6 of the associated detector 4.
  • a resistor 20 and a normally non-conductive diode 21, which are common to the three shunt-connected circuits 8, are connected across the lines 10 and 11 and across the lines 11 and 15 respectively.
  • the line 15 is connected in the circuit 9 to a voltage-level detector that involves two operational amplifiers 22 and 23.
  • the signal voltage developed across a resistor 24 connected between the lines 15 and 11 is applied to the amplifiers 22 and 23 via a resistor 25.
  • the amplifiers 22 and 23 act respectively to compare this voltage with upper and lower voltage levels which differ from one another by 0.25 volt and which are established in a voltage-divider chain of three resistors 26, 27 and 28 connected across lines 10 and 11.
  • the outputs of the two amplifiers 22 and 23 are connected via respective diodes 29 and 30 to the junction of resistors 31 and 32 in a voltage-divider chain which is connected across lines 10 and 11 and which includes two further resistors 33 and 34.
  • the junction of the resistor 32 with the resistor 33 in this chain is connected to the base electrode of an N-P-N transistor 35 that is connected in the common-emitter circuit configuration for supplying tamper-alarm signals to the control equipment 7. It is to the junction of the resistor 31 with the resistor 34, on the other hand, that line 14 is connected in the circuit 9.
  • the junction of the resistors 31 and 34 in the circuit 9 is established as a virtual earth (point of constant potential) by negative feedback from an operational amplifier 36.
  • the output of the amplifier 36 is in this respect connected to such junction via a resistor 37 that is shunted by a diode 38, whereas one of its inputs is connected to this same junction via a resistor 39.
  • the other input of the amplifier 36 and one of the inputs of a fourth operational amplifier 40 are connected to the junction of two resistors 41 and 42 that are connected in series chain with two diodes 43 and 44 across lines 10 and 11.
  • the amplifier 40 compares the output of the amplifier 36 with the potential at the junction of resistors 41 and 42 to derive intrusion-alarm signals for supply to the control equipment 7.
  • a low-output signal is also applied to the equipment 7 by the normally-conductive transistor 35, the transistor 35 in this respect remaining conductive while both diodes 29 and 30 are non-conductive. Both diodes 29 and 30 remain non-conductive while the potential of line 15 with respect to terminal 13 remains at its normal value within the upper and lower limits of comparison established for the amplifiers 22 and 23 in the chain of resistors 26 to 28.
  • the amplifier 36 responds to oppose the consequent tendency' for change in potential of line 14, by increasing current flow in the resistors 37 and 31.
  • the output signal of the amplifier 36 applied to the amplifier 40 therefore rises above the potential at the junction of resistors 41 and 42.
  • the amplifier 40 in its turn responds to this to provide a high-output signal to the equipment 7 indicative of the intrusion-alarm condition signalled by the opening of contacts 5.
  • opening of the set of contacts 5 of any detector 4 does not affect the magnitude of current flow in line 15, since the emitter current of the associated transistor 16 remains constant in spite of such opening. Accordingly the potential of line 15 is unchanged and the transistor 35 in the circuit 9 continues to conduct as normal.
  • opening of the contacts 6 in any of the detectors 4 breaks current flow to line 15 through the resistor 19 of the associated circuit 8. This break in current flow causes the potential of line 15 to fall below the lower comparison limit established for the amplifier 23.
  • the response of the amplifier 23 to this condition causes the diode 30 to conduct and this in its turn causes the transistor 35 to become non-conductive.
  • a high-output signal is thus applied in the equipment 7 from the collector circuit of the transistor 35 to indicate the tamper-alarm condition signalled by the opening of the contacts 6.
  • the tamper-alarm condition is also signalled in the system if there is tampering with the cable 2, or more particularly with any of lines 10, 11, 14 and 15. If line 10 is broken, this again cuts off current flow to line 15 through the resistor 19, reducing the potential of line 15 below the lower comparison limit and producing the high-output signal from the transistor 35 in the same manner as for the opening of contacts 6. Reduction in potential of line 15 to produce the same outcome also takes place if line 15 itself, or line 14, is broken; in the latter case the transistor 16 loses its base reference and current supply to line 15 via the resistor 18 is in consequence reduced significantly.
  • the system also responds to any of the conditions in which lines 10, 11, 14 and 15 are shorted together rather than being broken. More especially, shorting of line 14 to line 10 increases current flow to line 15 in each circuit 8 so that the potential of line 15 rises above the upper comparison limit for the amplifier 22. The same applies if line 14 is shorted to line 15 or line 15 is shorted to line 10, the response of the amplifier 22 in all three cases bringing about indication of the tamper condition to the equipment 7. Shorting of line 14 or line 15 to line 11 similarly brings about an indication of the tamper condition, the current flow in line 15 being in each case reduced with a consequent fall in potential of line 15 below the lower comparison limit for the amplifier 23. Finally, shorting together of lines 10 and 11 will be effective, in the blowing of fuses in the power-supply source to terminals 12 and 13 or otherwise, to indicate the tamper condition to the equipment 7 in the normal manner for power failure.
  • a capacitor 45 is connected with the resistor 39 to decouple high-frequency components on line 14 from the input to the amplifier 36.
  • a capacitor 46 is connected with the resistor 25 to decouple high-frequency components on line 1 from the inputs to the amplifiers 22 and 23.
  • the diodes 43 and 44 serve to ensure that the potential of the junction of resistors 41 and 42 and used as the reference for operation of the amplifiers 36 and 40, is appropriately related to the establishment of a potential of 6 volt positive with respect to terminal 13 for the emitter electrode of the transistor 16 in each circuit 8.
  • the voltage drop across each diode 43 and 44 is substantially equal to the base-to- emitter voltage of the transistor 16 and remains so with temperature change, thereby providing for temperature compensation in the operation of the amplifiers 36 and 40.
  • the system has been described above as including three detector units 1, more or fewer, and in particular just one, may in principle be provided.
  • a single resistor 20 is used it is preferably located at the furthest extremity of the lines 10, 11, 14 and 15.
  • the circuits 8 and 9 may be provided in the form of devices that can be readily added into existing systems to upgrade them, and in these circumstances each circuit 8 may incorporate the resistor 20 and diode 21; where more than one circuit 8 is to be installed in such a system the resistor 20 may then be disconnected in all but the most remote installation.

Landscapes

  • Business, Economics & Management (AREA)
  • Emergency Management (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Burglar Alarm Systems (AREA)
  • Alarm Systems (AREA)
EP19790300631 1978-04-17 1979-04-17 Electrical alarm circuits and systems Expired EP0005046B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB1507078 1978-04-17
GB1507078 1978-04-17

Publications (2)

Publication Number Publication Date
EP0005046A1 EP0005046A1 (en) 1979-10-31
EP0005046B1 true EP0005046B1 (en) 1982-02-17

Family

ID=10052507

Family Applications (1)

Application Number Title Priority Date Filing Date
EP19790300631 Expired EP0005046B1 (en) 1978-04-17 1979-04-17 Electrical alarm circuits and systems

Country Status (3)

Country Link
EP (1) EP0005046B1 (es)
DE (1) DE2962120D1 (es)
ES (1) ES479659A1 (es)

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4622541A (en) * 1984-01-09 1986-11-11 Napco Security Systems, Inc. Intrusion detection system
DE19505794C1 (de) * 1995-02-09 1996-08-08 Siemens Ag Schaltungsanordnung zur Übermittlung von wenigstens zwei Meldesignalen

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3997889A (en) * 1974-01-30 1976-12-14 Sharp Kabushiki Kaisha Alarm system for indicating a fault condition of an anti-skid braking control system
DE2621209B1 (de) * 1976-05-13 1977-07-28 Preussag Ag Feuerschutz Meldeeinrichtung,insbesondere zur Brandmeldung

Also Published As

Publication number Publication date
ES479659A1 (es) 1979-10-16
EP0005046A1 (en) 1979-10-31
DE2962120D1 (en) 1982-03-25

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